The Star Formation in Radio Survey: GBT 33 GHz Observations of Nearby Galaxy Nuclei and Extranuclear Star-Forming Regions

TL;DR

This study uses 33GHz radio observations from the GBT to evaluate star formation rates in galaxy nuclei and star-forming regions, demonstrating the robustness of 33GHz emission as a star formation indicator and exploring related empirical calibrations.

Contribution

It provides new high-resolution 33GHz measurements of galaxy regions, establishing the thermal fraction as a key indicator of star formation and analyzing its relation to other emission properties.

Findings

Median thermal fraction at 33GHz is ~76% in non-AGN sources.

Thermal fraction exceeds 90% in regions smaller than 0.5kpc.

The 33GHz emission reliably traces ionizing photon rates from young stars.

Abstract

We present 33\,GHz photometry of 103 galaxy nuclei and extranuclear star-forming complexes taken with the Green Bank Telescope (GBT) as part of the Star Formation in Radio Survey (SFRS). Among the sources without evidence for an AGN, and also having lower frequency radio data, we find a median thermal fraction at 33GHz of ~76% with a dispersion of ~24%. For all sources resolved on scales <0.5kpc, the thermal fraction is even larger, being >90%. This suggests that the rest-frame 33GHz emission provides a sensitive measure of the ionizing photon rate from young star-forming regions, thus making it a robust star formation rate indicator. Taking the 33GHz star formation rates as a reference, we investigate other empirical calibrations relying on different combinations of warm 24\mu m dust, total infrared (IR; 8-1000\mu m), H\alpha\ line, and far-UV continuum emission. The recipes derived here generally agree with others found in the literature, albeit with a large dispersion that most likely stems from a combination of effects. Comparing the 33GHz to total IR flux ratios as a function of the radio spectral index, measured between 1.7 and 33GHz, we find that the ratio increases as the radio spectral index flattens which does not appear to be a distance effect. Consequently, the ratio of non-thermal to total IR emission appears relatively constant, suggesting only moderate variations in the cosmic-ray electron injection spectrum and ratio of synchrotron to total cooling processes among star-forming complexes. Assuming that this trend solely arises from an increase in the thermal fraction sets a maximum on the scatter of the non-thermal spectral indices among the star-forming regions of \sigma_\alpha^{NT} < 0.13.